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Is this the missing link for this build?
Based on some of the other instructables, the signal seems to be a 7.8kHz pulse, which is smack in the audible range of frequencies (or VLF - Very Low Frequency - in the EM spectrum), so it can easily be captured on an mp3 player.
All that's needed is some sort of antenna to transmit the signal. For a VLF signal like this, a telescopic antenna isn't ideal, but it's better than nothing. Remember - the store probably just uses a long wire buried around the edges of the parking lot. You may want to put a resistor in series with the antenna to avoid shorting the output amp.
Don't know whether a speaker would work better - speaker are designed to transduce an electrical signal to a sound signal, not to emit a strong electromagnetic field. A large induction coil such as used in some of the other instructables would work much better.
but, does the volume effect the magnetic field power such as higher sound level equals better chance or better radius of locking up?